Ian McKellen in The Hobbit: An Unexpected Journey
It’s almost impossible to imagine the role of Gandalf being played by anyone other than Sir Ian McKellen, who put his indelible stamp on the character in all six Lord of the Rings and Hobbit films. But if things had worked out a little differently during the filming of The Hobbit: An Unexpected Journey, director Peter Jackson might have had to find a different actor for the part.
The problem? McKellen was acting almost exclusively to a green screen, owing to the fact that his character towered over the hobbits and their interactions had to be stitched together digitally in production. The frustration of acting alone eventually got to McKellen, who broke down on the set at one point, but Jackson did a little emergency outreach to convince him to stick around.